The first flight of the Hopkins Ultraviolet Telescope (HUT)
aboard the Astro-1 space shuttle mission in 1990 December clearly
demonstrated the value of far-ultraviolet spectrophotometry for
many diverse areas of astrophysical research ([Davidsen 1993]). HUT
is designed to observe faint sources at moderate resolution (3 Å) in
the (first order) wavelength range 820--1840 Å. It thus overlaps
with the spectroscopic capabilities of the Hubble Space Telescope
( HST) at 
Å, and provides an important complementary
capability for studies of the little-explored, but astrophysically
interesting 912--1200 Å region. By employing just two reflections,
special optical coatings, and a windowless detector system, HUT
achieves a sensitivity equal to that of the Faint Object Spectrograph
(FOS) on HST at about 1200 Å ([Keyes et al. 1995]), and maintains
high sensitivity down to the Lyman limit ([Davidsen et al. 1992]).
Prior to the flight of Astro-1, spectroscopic studies at far-UV wavelengths
were essentially limited to high resolution work on very bright stars with
Copernicus ([Rogerson 1973]) and very low resolution observations
with Voyager ([Holberg 1991]). More recently there has been
some further work at
these wavelengths with ORFEUS ([Grewing et al. 1991]; [Hurwitz & Bowyer 1991]).
HUT carried out the first extensive moderate resolution work
at wavelengths from Lyman-
to the Lyman limit.
The success of HUT on Astro-1 was tempered somewhat by the
relatively short mission duration (9 days) and by difficulties
encountered with the Spacelab Instrument Pointing System (IPS).
NASA's decision to re-fly HUT on the Astro-2 mission provided the
opportunity to correct these deficiencies and also to make significant
improvements in HUT's performance which are described here. A factor of 2.3
increase in sensitivity was achieved, primarily through replacing the
primary mirror and diffraction grating with new ones coated with
previously-unavailable, ion-sputtered silicon carbide. IPS hardware
and software problems were corrected, resulting in excellent pointing
performance, with 
stability obtained routinely. An
extended-duration mission totalling 16 days, the longest space
shuttle flight conducted to date, enabled us to capitalize fully on
these improvements.
A guest investigator program for Astro-2 expanded the scope of
scientific topics addressed with HUT. Altogether, about 20 scientific
programs were successfully conducted. We obtained 385 observations of
265 different celestial targets, with a total on-source integration
time of 205 hours.
The excellent
performance of HUT/Astro-2 was highlighted by our success in observing
the quasar HS 1700+64 (
, 
, 
), toward
which the opacity of singly-ionized intergalactic helium was measured
([Davidsen, Kriss & Zheng 1995]). In this Letter we describe the
changes made to HUT and its performance on Astro-2, including
a preliminary, but detailed calibration.